Clean up duplicate codes in Plan implementation of ET and Cruise-PS

[wynot12]

Since #685, ET and Cruise-PS have separate plan implementations. ET's one is for considering only ADD/DEL/MOVE operations, and Cruise-PS's one is extended to handle START/STOP operations, which control the tasks running on workers.

Both implementations perform similar tasks, but they have separate code. For the moment, it looks that it's not easy to unify them and make them share the same code.

But we may improve the code by removing duplicate code, if we have better idea in future.

Note that unit tests for them have also duplicate code.